In machining with exchangeable and/or indexable cutting inserts there is an increased need to measure on-line the changes of condition that the cutting insert is subjected too close to the cutting zone thereof during operation. On basis of such on-line measurement of the condition of the cutting insert, decisions regarding altering of operation parameters, exchange of cutting insert or repositioning of the cutting insert in its holder can be taken. Time-requiring manual inspection of the condition of the cutting insert can be avoided and, as a consequence thereof, an improved efficiency and a more automated operation can be obtained. Furthermore, a precise on-line measurement of the condition of the cutting insert, and correct measures taken in the right time as a consequence thereof, will help to prevent damaging of the work piece due to use of excessively worn cutting inserts or too high temperatures.
Prior art as disclosed in EP1186365B1 and EP 1095732B1 describes cutting inserts in which a sensor for on-line measurement of the wear of the cutting insert comprises an electric circuit formed by a conductive layer provided on the cutting insert, and wherein the electric circuit comprises at least two contact regions through which the electric circuit is connectable to external measuring circuitry. A cutting insert holder presents corresponding electrical contacts which are electrically connected to a respective of the contact regions of the cutting inserts as the cutting insert is held in an operative position by the tool holder. Connected to the contacts of the tool holder there is provided measuring circuitry for measuring of a change of resistance of the electric circuit caused by a predetermined wear of the cutting insert. The measuring circuitry comprises means for generating an electric current in the electric circuit provided on the cutting insert during operation thereof. A lead, or sensor line, of the electric circuit is positioned on a clearance face of the cutting insert in close proximity to a cutting edge of the cutting insert and extends along the cutting edge. The lead of the electric circuit is positioned such that, upon a predetermined wear of the cutting edge, the lead will be in engagement with the work piece, and the resistance of the electric circuit as measured by means of said external measuring circuitry will change. At a predetermined critical degree of wear of the cutting insert, the lead will be cut, indicating that the cutting insert must be exchanged or be given another position in the holder. On basis of the wear down of the lead and finally the cutting off thereof, it is thus detected how far the wear of the cutting insert has proceeded, and, depending thereon, measures such as exchange of the cutting insert may be taken.
However, with the suggested solution as disclosed by EP1186365B1 and EP 1095732B1, the signal obtained in the external circuitry, indicating that the wear down of the cutting insert has proceeded to a certain point, may not always be so stable due to fluctuating engagement between the work piece and the lead of the electric circuit of the cutting insert.
U.S. Pat. No. 4,744,241 discloses a method and an arrangement for detecting a wear limit or a break in a cutting edge of a machine tool. At least two conductor paths are embedded in the cutting material forming the cutting edge, one of the conductor paths being part of a closed circuit and the other of the conductor paths being part of an open circuit. A signal serving to break off the machining process is produced if either the conductor path of the closed circuit is interrupted or the conductor path of the open circuit is closed by the creation of a conductive connection between the two conductor paths. The conductor path of the closed circuit is arranged along the cutting edge of the cutting insert so as to be interrupted by a crack propagating in a region of a secondary cutting edge of the latter, as a result of a predetermined wear of the cutting insert. However, in cutting processes, a conductive bridge may be formed across a crack by particles of the material of the work piece or the coolant or by particles of an electrically conductive cutting substance, so that, in spite of the fact that the limit of permissible wear has been reached or a break in the cutting edge has occurred, the signal for breaking off the machine is not initiated by any signal from the interrupted closed circuit. In order to cope with that drawback U.S. Pat. No. 4,744,241 suggests the provision of the above-mentioned conductor path of an open circuit which is positioned in parallel with and arranged to be electrically connected to the conductor path of the closed circuit upon a predetermined degree of wear of the cutting insert. Upon such interconnection, a signal for breaking off the machining process is assumed to be initiated. However, since the conductor path of the closed circuit is arranged so as to be interrupted by a crack, and might in fact be interrupted by one or more such cracks at the moment when that conductor path is connected to the conductor path of the open circuit, also this signal for breaking off the machining process might not be initiated. The signal received from any of the circuits runs the risk of being non-distinct since the conductor path of the closed circuit may be interrupted by a crack but may be temporarily closed through the above-described formation of a conductive bridge across the crack or cracks. It will thus be unclear whether a predetermined wear of the cutting insert has occurred or not, and the indication of the predetermined wear becomes unreliable.
Moreover, the position of the crack or the wear down of the cutting insert cannot be determined on basis of signals received from the above-mentioned circuits.
It is an object of the present invention to present a cutting insert that enables a more precise and reliable detection of a predetermined wear of a cutting insert caused by operation of the cutting insert on a metal work piece.